The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electromechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque thereto. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed. A hydraulic control system is known to provide pressurized hydraulic oil for a number of functions throughout the powertrain.
Operation of the above devices within a hybrid powertrain vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and driveline. Input torque from the engine and input torque from the electric machine or electric machines can be applied individually or cooperatively to provide output torque. Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage to and disengage the various components from the transmission in order to perform the functions of the hybrid powertrain system. Engagement and disengagement are known to be accomplished within the transmission by employing selectively operable clutches.
Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque differences between the shafts. Clutches are known in a variety of designs and control methods. One known type of clutch is a mechanical clutch operating by separating or joining two connective surfaces, for instance, clutch plates, operating, when joined, to apply frictional torque to each other. One control method for operating such a mechanical clutch includes applying the hydraulic control system implementing fluidic pressures transmitted through hydraulic lines to exert or release clamping force between the two connective surfaces. Operated thusly, the clutch is not operated in a binary manner, but rather is capable of a range of engagement states, from fully disengaged, to synchronized but not engaged, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. The clamping force available to be applied to the clutch determines how much reactive torque the clutch can carry before the clutch slips.
The hydraulic control system, as described above, utilizes lines charged with hydraulic oil to selectively activate clutches within the transmission. However, the hydraulic control system is also known to perform a number of other functions in a hybrid powertrain. For example, an electric machine utilized within a hybrid powertrain generates heat. Known embodiments utilize hydraulic oil from the hydraulic control system in a continuous flow to cool the electric machine in a base machine cooling function. Other known embodiments additionally are known to react to higher electric machine temperatures with a selectable or temperature driven active machine cooling function, providing additional cooling in the high temperature condition. Additionally, known embodiments utilize hydraulic oil to lubricate mechanical devices, such as bearings. Also, hydraulic circuits are known to include some level of internal leakage.
Hydraulic oil is known to be pressurized within a hydraulic control system with a pump. The pump can be electrically powered or preferably mechanically driven. In addition to this first main hydraulic pump, hydraulic control systems are known to also include an auxiliary hydraulic pump. The internal impelling mechanism rotates operates at some speed, drawing hydraulic oil from a return line and pressurizing the hydraulic control system. The supply of hydraulic flow by the pump or pumps is affected by the speed of the pumps, the back pressure exerted by the hydraulic line pressure (PLINE), and the temperature of the hydraulic oil (TOIL).
The resulting or net PLINE within the hydraulic control system is impacted by a number of factors. FIG. 1 schematically illustrates a model of factors impacting hydraulic flow in an exemplary hydraulic control system, in accordance with the present disclosure. As one having ordinary skill in the art will appreciate, conservation of mass explains that, in steady state, flow entering a system must equal the flow exiting from that system. As applied to FIG. 1, a flow of hydraulic oil is supplied to the hydraulic control system by the pumps. The flow exits the hydraulic control system through the various functions served by the hydraulic control system. This exemplary embodiment includes the following functions: hydraulic oil fills clutch mechanisms in order to provide clamping force required to lock the clutch, as described above; hydraulic oil provides both base cooling and active cooling of the electric machines and other components as required; hydraulic oil is used to lubricate portions of the transmission; and hydraulic oil flows through leakage internal to the hydraulic circuit. PLINE describes the resulting charge of hydraulic oil maintained in the system: for any flow through a system, the resulting pressure within the system depends upon the flow resistance within the system. Higher flow resistance in the system results in higher system pressures for a given flow. Conversely, lower flow resistance in the system results in lower system pressures for a given flow. Applied to FIG. 1, PLINE or the pressure within the hydraulic control system, changes depending upon usage of the hydraulic control system. For example, filling a previously unfilled transmission clutch consumes a significant amount of hydraulic oil from the hydraulic control system. The orifice leading to the clutch includes low resistance in order to draw the significant amount of hydraulic oil over a short time span. As a result, during the clutch filling process, PLINE in an otherwise unchanged hydraulic control system will reduce. Conversely, for a given set of functions served by the hydraulic control system, PLINE varies based upon the flow supplied by the pumps. For any given set of flow restrictions associated with the functions served, increased flow from the pumps will result in higher PLINE.
As noted above, main hydraulic pumps are known to be powered mechanically, driven as a parasitic device from the engine. Hybrid powertrains are known to operate with an engine running or stopped, depending upon the current hybrid control strategy. Under engine stopped operation in a powertrain utilizing a mechanically driven main pump, the main pump cannot provide a supply of hydraulic flow, and, instead, an auxiliary pump must be used to provide PLINE required to operate the vehicle. A method to accurately control PLINE in a hybrid powertrain through engine running and stopped operation would provide useful control of the hydraulic control system.